Definitions for "Complex System"
These are systems with a large number of interrelated parts. Thus the term is somewhat subjective and interpretive.
A block diagram that cannot be reduced to series and/or parallel systems.
Spatially and/or temporally extended nonlinear systems characterized by collective properties associated with the system as a whole--and that are different from the characteristic behaviors of the constituent parts.(From the FAQ sheet of the sci.nonlinear newsgroup)